Methods and systems for processing materials, including shape memory materials

ABSTRACT

A method for treating a material comprising: applying energy to a predetermined portion of the material in a controlled manner such that the local chemistry of the predetermined portion is altered to provide a predetermined result. When the material is a shape memory material, the predetermined result may be to provide an additional memory to the predetermined portion or to alter the pseudo-elastic properties of the shape memory material. In other examples, which are not necessarily restricted to shape memory materials, the process may be used to adjust the concentration of components at the surface to allow the formation of an oxide layer at the surface of the material to provide corrosion resistance; to remove contaminants from the material; to adjust surface texture; or to generate at least one additional phase particle in the material to provide a nucleation site for grain growth, which in turn, can strengthen the material.

The present application claims priority from U.S. Provisional PatentApplication No. 61/232,243 filed Aug. 7, 2009 and from U.S. ProvisionalPatent Application No. 61/292,367 filed Jan. 5, 2010, both of which areincorporated herein by reference.

FIELD

The present document is related to processing of materials, includingmetals, alloys and shape memory materials. Shape memory materialsinclude shape memory alloys (SMA) and shape memory polymers (SMP). Inparticular, the present document relates to methods and systems forprocessing or treating materials to adjust the local chemistry of apredetermined area in a controlled manner to achieve a predeterminedresult.

BACKGROUND

Material processing is used in almost every industry to producematerials of varying properties for products of varying application. Insome areas, methods of material processing are still developing. Thisincludes the area of shape memory materials.

Shape memory materials are materials that can be trained to hold andreturn to a particular shape when at a higher temperature and bemalleable at a lower temperature. Even if bent into a different shapewhen at the lower temperature, the material returns to the trained shapewhen the temperature is raised. The temperature at which the materialreverts back to the trained high temperature configuration is typicallyreferred to as the transformation temperature. The shape memory effectthat occurs in these materials is related to a reversible solid statephase transition in which the material transforms between an austeniticstate and a martensitic state with a decrease in temperature. In themartensitic state, the shape memory material becomes more easilydeformed and is typically able to accommodate significant plasticdeformation at an almost constant stress level. When the shape memorymaterial is in the martensitic state, it can be heated and theapplication of heat results in the metal returning to the austeniticstate. The transformation may occur at a particular temperature or overa range of temperature. Shape memory materials have become quite wellknown and are used in many applications such as medical (e.g. stents),industrial, automotive, aerospace and various others.

Shape memory materials can be generally divided into shape memorymetals/alloys (SMAs) and shape memory polymers (SMPs). Many alloys maybe manipulated into a shape memory material, including some magneticmaterials and alloys. Three main types of SMAs include:

1) Nickel-titanium (NiTi)

2) Copper-Zinc-Aluminum-Nickel

3) Copper-Aluminum-Nickel

Other SMAs include, but are not limited to, the following:

1) Ag—Cd 44/49 at. % Cd

2) Au—Cd 46.5/50 at. % Cd

3) Cu—Al—Ni 14/14.5 wt. % Al and 3/4.5 wt. % Ni

4) Cu—Sn approx. 15 at. % Sn

5) Cu—Zn 38.5/41.5 wt. % Zn

6) Cu—Zn—X (X═Si, Al, Sn)

7) Fe—Pt approx. 25 at. % Pt

8) Mn—Cu 5/35 at. % Cu

9) Fe—Mn—Si

10) Pt alloys

11) Co—Ni—Al

12) Co—Ni—Ga

13) Ni—Fe—Ga

14) Ti—Pd in various concentrations

15) Ni—Ti (˜55% Ni)

(at. %=atomic percent)

Examples of SMPs include, but are not limited to, the following:

-   -   1) Polyurethane-based shape-memory polymers with ionic or        mesogenic components    -   2) Polyethylene-terephthalate-Polyethyleneoxide (PET-PEO) block        copolymer crosslinked using Maleic Anhydride

One of the most common shape memory materials is nitinol (sometimesreferred to as NiTi), an alloy of nickel and titanium. This applicationfocuses on SMAs and nitinol in particular, however, similar principlescan apply to other SMAs, SMPs or shape memory materials, as will beunderstood by one skilled in the art.

SMAs are typically monolithic materials that are capable of a singletransformation temperature. The physical properties of SMA's, includingelasticity and stiffness, are affected by a variety of factors includingthe chemical composition of the SMA and the particular treatment towhich the SMA is subjected. In particular, for a nitinol SMA havingslightly varying near-equiatomic base metal compositions, the ratio ofNI to TI can significantly affect the transformation temperature.

The excellent pseudoelasticity, shape memory and biocompatibility ofnitinol have made it a leading candidate for various applications,including aerospace, micro-electronics and medical devices. Itspseudoelastic properties enable nitinol to experience up to 18% strainand subsequently fully recover upon release. The shape memory effectresults from nitinol's ability to transform from a rigid hightemperature austenite phase to a malleable low temperature martensitephase during cooling. Once a high temperature shape is trained into anitinol workpiece in the austenite phase, it can then be cooled to itsmartensite phase and be elastically deformed; however upon heating, thematerial will transform back into the austenite phase and return to itsoriginal shape. Primary factors affecting the transformation temperatureinclude 1) alloying elements (i.e. the Ni to Ti ratio), 2)thermo-mechanical processing and 3) precipitates embedded in the metalmatrix.

While the properties of nitinol with one transformation temperature arequite well known, more recently, efforts have been made to producemonolithic nitinol that has more than one transformation temperature inorder to broaden the range of applications for SMAs and to make themmore useful in existing applications.

The applicants are aware of two material forming techniques underdevelopment that are intended to be used to form monolithic shape memoryalloys from base elements to provide an SMA having multipletransformation temperatures.

1) Tape Casting utilizes varying compositions of elemental powders andsinters them to form a monolithic material. Sintered near equi-atomicnickel and titanium powders have recently exhibited shape memoryeffects. Furthermore, attempts to vary local compositions on amonolithic sheet have been demonstrated. However the inherent nature oftitanium to oxidize makes it extremely difficult to control the actualcomposition and the process can form a brittle structure. In addition,the porous material formed during sintering generally results in poormechanical properties.

2) Laser engineering net shaping (LENS) is a commercially availablerapid prototyping process, which uses elemental powders to create alayer by layer structure. By varying process parameters, it may bepossible to modify transformation temperatures during processing.However, complexities associated with processing can make it difficultto accurately tailor transformation temperatures. In addition, the finalproduct typically has a coarse surface finish and can requireconsiderable post-processing.

Based on the foregoing, there is a need for improved methods and systemsfor processing or treating materials and, in particular, shape memorymaterials in order to provide a material with multiple transformationtemperatures and attempt to overcome at least some of the concernsdescribed above.

SUMMARY

According to one aspect herein, there is provided a method for treatinga material comprising: applying energy to a predetermined portion of thematerial in a controlled manner such that the local chemistry of thepredetermined portion is altered to provide a predetermined result.

In applying the energy in a controlled manner, it is possible to treatonly a portion of the material while leaving other portions of thematerial generally unaffected and also allows for more complexadjustment of the local chemistry and structure. In the context of anSMA, this allows a memory or additional memory to be placed in thematerial at a predetermined position and having a generallypredetermined transformation temperature. It will be understood that, insome cases, the predetermined portion may include all of the material.

In a particular case, the applying energy comprises processing thepredetermined portion with a laser. In this case, the method mayinclude: selecting a power, beam size, and movement speed for the laserto produce the predetermined result; focusing the laser on a subset ofthe predetermined portion; and adjusting the spatial relationship of thelaser and the material such that a beam from the laser contacts all ofthe predetermined portion. In some cases, the laser may be operated in apulsed fashion to provide shorter bursts of energy to control theapplication of energy.

As noted above, the applied energy is generally controlled to reduceconduction outside the predetermined portion of the material.

In various particular cases, the predetermined or desired result mayvary depending on the desired use/application for the material and thematerial properties.

For example, when the material is a shape memory material, thepredetermined result may be to provide an additional memory to thepredetermined portion of the shape memory material (i.e. provide atransformation temperature to the predetermined portion that isdifferent from the transformation temperature of the remainder of thematerial) or to alter the pseudo-elastic properties of the shape memorymaterial to provide additional pseudo-elastic regions.

In other examples, which are not necessarily restricted to shape memorymaterials, other results may be intended.

For example, the predetermined portion may be the surface or surfacelayer of the material and the predetermined result is to adjust theconcentration of components in the surface or surface layer to allow theformation of an oxide layer at the surface of the material to providecorrosion resistance. It will be understood that the depth of thesurface layer will depend on material properties, method of energyapplication, intended use of the material and the like.

In another example, the predetermined result may be to removecontaminants from the material.

In yet another example, the predetermined result may be to generate atleast one additional phase particle in the material. The formation ofparticles in an additional phase can provide a nucleation site for graingrowth, which in turn, can strengthen the material.

In some cases, the cooling of the predetermined portion of the materialmay also be controlled to produce a predetermined result. For example,the predetermined portion may be cooled at a predetermined rate to alterthe surface texture of the predetermined portion.

In yet a further case, the method may include adding a filler materialsuch that the filler material is available during the application ofenergy. In this case, additional quantities of a component of thematerial may be added to alter the composition (e.g. concentration ofspecific components) of the predetermined portion or other materials maybe added to affect the local chemistry of the predetermined portion inother ways.

In still yet a further case, the material comprises two pieces of shapememory material and the predetermined portion comprises an area wherethe two pieces are to be bonded and the predetermined result comprisesproviding a transformation temperature to the predetermined portion thatis different from a transformation temperature of at least one of thepieces.

According to another aspect herein, there is provided a shape memorymaterial comprising at least two transformation temperatures wherein atleast one transformation temperature is applied following formation ofthe material. In a particular case, at least one of the at least twotransformation temperatures are formed by the method described above.

According to yet another aspect herein, there is provided a system fortreating a material comprising: an energy module for applying energy toa predetermined portion of the material; a position module forpositioning the material and energy module in relation to each other;and a processing module for controlling the position module and energymodule to treat the material such that the local chemistry of thepredetermined portion of the material is altered to provide apredetermined result.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the embodiments described herein and toshow more clearly how they may be carried into effect, reference willnow be made, by way of example only, to the accompanying drawings whichshow example embodiments and details and in which:

FIG. 1 is a flow chart of an embodiment of a method for processing amaterial to alter the local chemistry in a controlled fashion;

FIG. 2 is a block diagram of an embodiment of a system for processing amaterial to alter the local chemistry in a controlled fashion;

FIG. 3A illustrates conduction welding;

FIG. 3B illustrates keyhole welding;

FIGS. 4A and 4B are schematics showing dimension of tensile specimens;

FIG. 5 illustrates a loading-unloading curve for pseudoelastic NiTiallow;

FIGS. 6A and 6B illustrate the effects of process parameters on minimumweld width;

FIG. 7 is a representative tensile curve for varying pulse frequency;

FIG. 8 is a representative tensile curve for varying peak power input;

FIG. 9 illustrates a view of multiple plateaus in welded samples;

FIG. 10 illustrates a first and a second loading curve;

FIGS. 11A and 11B show cyclic loading of unwelded and laser weldedspecimen;

FIGS. 12A and 12B illustrate the micro-hardness trace along a verticaland horizontal weld axis;

FIG. 13 illustrates differential scanning calorimetry (DSC) scans forbase and weld material;

FIGS. 14A and 14B show optical micrographs of base material and fusionboundary microstructure;

FIGS. 15A, 15B and 15C show X-ray diffraction data (XRD) for weld topand bottom of 0.6 kW, 10 pps, 0.6 kW, 1 pps and 0.9 kW, 10 ppsrespectively as compared to base metal;

FIGS. 16A, 16B and 16C are photographs showing a nitinol ribbon withmultiple transformation temperatures;

FIGS. 17A and 17B are illustrations of additional shapes of SMAs, havingtwo dimensional or three dimensional application of differingtransformation temperatures;

FIG. 18 shows an additional illustration of the application of differingtransformation temperatures to a shape memory element;

FIG. 19 illustrates an anticipated stress-strain curve for a strip ofshape metal material having multiple transformation temperatures;

FIG. 20 shows an example application of a shape memory element havingmultiple transformation temperatures;

FIG. 21 shows another example application of a shape memory elementhaving multiple transition temperatures;

FIGS. 22A and 22B show another example application of a shape memoryelement having multiple transition temperatures;

FIG. 23 illustrates a cross-section showing bulk material that includescontaminants;

FIG. 24 illustrates Ti₂Ni particles in nitinol following processing;

FIG. 25A to 25D illustrate the effects of composition change over aseries of laser pulses; and

FIG. 26 is a phase diagram illustrating the second phase transition.

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where considered appropriate, reference numerals may be repeated amongthe figures to indicate corresponding or analogous elements or steps. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the exemplary embodiments described herein.However, it will be understood by those of ordinary skill in the artthat the embodiments described herein may be practiced without thesespecific details. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure theembodiments described herein. Furthermore, this description is not to beconsidered as limiting the scope of the embodiments described herein inany way, but rather as merely describing the implementation of thevarious embodiments described herein.

While the discussion below focuses to some extent on shape memory alloys(SMAS), it will be understood that the principles, processes and systemscan be similarly applied to other shape memory materials. Further, as aninteresting result, the process initially developed in relation to shapememory materials can also have some application to other materials,including metals, as described below.

Traditional shape memory alloys (SMA) are batch processed to produce amonolithic sheet having a single transformation temperature. Thisprocessing is most appropriate due to the homogeneous composition andstructure within the SMA. Thus, this processing only allows the SMA tohave a single transformation temperature for a given “remembered” shape.

As noted above, attempts have been made to fabricate an SMA having morethan one transformation temperature. In order to examine the possibilityof joining two pieces of SMA (nitinol) having differing transformationtemperatures, the applicants herein have conducted testing on nitinolusing welding techniques. In particular, in order to examine thepotential for welding of two monolithic pieces of nitinol the applicantsconducted tests using a “bead on plate” process in which a monolithicsheet of nitinol (nitinol workpiece) was subject to a welding laser at acentral point of the monolithic sheet. During the process, energyapplied and thus temperatures sometimes exceeded those used inconventional welding processes. Interestingly, at higher temperatures,the effect of the laser was to melt a targeted area of the nitinol insuch a way that a local portion of the nitinol was fully melted (thatis, the nitinol underwent a phase change) but held in place by thesurface tension of the molten nitinol. Although in some cases anadditional intermediate phase transformation (such as R-phase innitinol) was encountered, the additional intermediate phase did not playappear to play a significant role in the shape memory effect discussedin further detail below.

During the local application of the laser, local temperatures andpartial pressure effects cause the melting and, it is believed, boilingof the material or constituents thereof. Although not anticipated at thetime, subsequent testing indicated that the portion of the nitinolworkpiece that was subject to the laser treatment exhibited a change inthe transformation temperature for that portion/area that was treated.It appeared that the melting of the nitinol and subsequentsolidification caused a change in the local chemistry of the nitinol.Consequently the processed area exhibited an additional memory while theremaining untreated material still exhibited its original properties andmemory. This unexpected development provided the background for thesystems and methods for treatment described in more detail herein. Inparticular, the systems and methods allow one or more additionalmemories to be embedded into a monolithic shape memory material sheet.As will be understood, having additional memories enables additionalfunctionality for many applications.

It is believed that the change in the transformation temperature isbecause the transformation temperature is very sensitive to the localstructure and chemistry of the nitinol. Because of vaporization duringmelting (due to temperature and the partial pressures involved), theprior microstructure is destabilized until the point where the moltenmetal subsequently re-solidifies. In particular, in the case of nitinol,the original base material for NiTi is typically a homogeneousstructure, which is saturated with either Nickel (when Ni is greaterthan 50 at. % (atomic percent)) or Titanium (when Ti is greater than 50at. %). This structure is usually attained by annealing the alloy(between 500 and 1200 degrees Celsius) then quenching to retain the NiTistructure. In a particular case, annealing the alloy may be accomplishedat approximately 800 degrees Celsius. Further, mechanical processing,such as rolling, may be conducted to refine the microstructure and addstrength. However, when the structure is melted and re-solidified (forexample, using a laser as described in further below) one or moreconstituents may be vaporized while the remaining saturated constituentsare pushed along with the solidification front with the final liquid tosolidify being rich in that particular chemistry. This local area willthen stabilize into an intermetallic (I.e. Ni rich: Ni₃Ti Ni₄Ti₃; TiRich: Ti₂Ni). This result may occur when there is an imbalance incomposition and there may be other mechanisms involved as well. Althoughthe overall chemistry of the re-solidified metal is generally the same(including matrix and intermetallic), the matrix chemistry will bedifferent from the original base material. Hence, the matrix in thelocal area will have a different transformation temperature.Interestingly, in some cases, peak temperatures can remain high longenough that the local area also experiences some degree ofpost-processing heat treatment (such as annealing), which may includethe heat affected zone.

The local melting of the SMA contrasts with some lower temperature formsof heat treatment of alloys/metals, such as annealing, because theselower temperature processes will have less impact on the internalstructure and chemistry as they occur in the solid state rather than ina molten liquid state. Further, when conducted appropriately, themelting process does not result in the complete destruction of thesuper-elasticity of the SMA when in the martensitic state, although itmay result in a change in the super-elasticity. Still further, theprocess can be performed on existing materials in contrast to processesthat are used to form SMAs from base constituents, as noted above.

Based on this unexpected information, the applicants herein havedeveloped methods and systems for processing/treating materials to alteror change the local chemistry/structure in a region to achieve apredetermined result. One particular result is to provide a shape memorymaterial, such as the SMA, nitinol, with altered properties and, inparticular, multiple transformation temperatures in differing zones ofthe monolithic material.

FIG. 1 shows a flow chart of an example method 100 of treating/forming amonolithic sheet or workpiece of nitinol having multiple transformationtemperatures. It will be understood that this method may be adapted toprocess other materials to alter the local chemistry/structure toprovide desired results, as described in further detail below.

The process 100 starts with the input of a monolithic sheet of nitinol.The monolithic sheet or workpiece of nitinol may first be processed toimpart a particular shape memory into the monolithic sheet 110. Theprocessing of the nitinol to impart a first shape memory (andtransformation temperature) is well known in the art. However, anunprocessed alloy having sufficient composition to exhibit the shapememory effect may also be processed, in which case a first memory willbe embedded using the process 100. The treated nitinol workpiece is thenmoved to a processing station where it is positioned for lasertreatment.

The method may include the use of a processor or the like toautomatically calculate the process parameters to be used based on thedesired transformation temperature, chemical composition orpredetermined result of the processing 120. An example of the types ofinformation, including transformation temperatures as a function of NiTichemistry and the like, that can be used in the calculation or inlook-up tables or the like is given in, for example, Tang W,Thermodynamic Study of the Low-Temperature Phase B19′ and theMartensitic Transformation in Near-Equiatomic Ti—Ni Shape Memory Alloys,Metallurgical and Materials Transactions A, Volume 28A, March, 1997, pp.537-544. It will be understood that this aspect of the embodiment mayconsist of computer readable instructions on a physical media that, whenperformed by a computing device (processor), cause the procedure to beperformed.

The nitinol workpiece is then subjected to laser treatment 130 in anarea that is intended to have the local chemistry altered, in this case,to provide a different transformation temperature. It will be understoodthat, depending on the application, a laser may be moved to ensure thatthe required area of the nitinol workpiece is laser treated, oralternatively, the nitinol workpiece may be moved in relation to thelaser. In the laser treatment 130, energy is applied to a local area ofthe nitinol such that at least some melting and vaporization occurs(based on the temperature and partial pressures at the local area). Therange of melting points for SMAs such as nitinol is affected by thechemical makeup of the SMA as well as chemical changes that may occur inthe heating process. The rate of vaporization is also affected by localpressure as is known in the art. For nitinol, some effect may beavailable after heating to a range of approximately 1,000 degreesCelsius and higher. This temperature range contrasts with some lowertemperature forms of heat treatment of alloys/metals, such as annealing,because these processes will have less impact on the internal structureas they occur in the solid state rather than in a molten liquid state.In further particular cases, the nitinol may be heated to betweenapproximately 1,250 and 1,280 degrees Celsius. In another case, thenitinol is heated to approximately 1,300 degrees Celsius or higher, forexample in the range of approximately 1,320 or 1,340 degrees Celsius.Generally speaking, the temperature is selected in order to provide asufficient level of melting and vaporization to occur such that thelocal chemistry is changed to provide the desired result, such as anadditional transformation temperature.

The application of energy to generate the heating is preferablylocalized and configured so that the change in the local chemistry willbe localized and there will not be any undesired spread of the effectinto other areas of the SMA sheet. In many cases, a shorter energyapplication process may provide a better defined area or zone ofdistinct change in local chemistry, and thus localized change intransformation temperature. As such, laser melting is preferred butother forms of heating such as resistance or plasma melting may also beused. In the case of laser melting, the appropriate temperature cantypically be reached in as little as one millisecond or less in order tohave very rapid heating and treatment of the SMA. In a particular case,the appropriate temperature may be reached in less than half amillisecond. Even with resistance or plasma heating the time of heatapplication can be as little as one second or less.

The energy application process, whether by laser heating or otherwise,will generally be performed in the presence of a shielding gas, such asArgon or similar known production gas. A shielding gas is used becausethe components or the shape memory material may react with oxygen toproduce unwanted by-products.

Cooling and re-solidification of the treated material will occur quicklyafter the removal of the energy source. Process parameters can beconfigured to provide controlled in-situ cooling rates. In some cases,the nitinol workpiece may be subject to further processing 140, forexample, cooling and re-solidification can be controlled by using a heatsink for more rapid cooling (i.e. copper block as a chiller or a coldgas) or a heated stage for slower cooling rates. Additional processingmay include further heat treatment as described in one example below orother processing to be prepared for a particular application.

FIG. 2 is a block diagram of an embodiment of a system for processingmaterial to produce a controlled change in local chemistry/structure. Inthis particular embodiment, the system is for forming an SMA havingmultiple transformation temperatures. The system includes a feed module150 for providing a nitinol workpiece to a positioning module 160 foradjusting the position of the nitinol workpiece prior to and/or duringtreatment, and a removal module 170 for moving the nitinol workpiece forfurther processing. The system also includes a heating/melting module180 that applies energy to the appropriate area of the nitinol workpiecebeing held at the positioning module 160. As described herein, theheating/melting module 180 may include a laser or otherdevices/materials for applying energy (typically heat). The system alsoincludes a shield gas module 190 that provides a shield gas to preventunwanted reactions during the heating/melting process. In someembodiments, the system may also include a processing module 200 thatcan be used to control the positioning module 160 and theheating/melting module 180 based on input parameters or automaticallycalculated values based on input parameters. The input parameters mayrelate to the type of processing to be performed and/or to the desiredresult.

It will be understood that the methods and systems described herein maybe performed at one or more processing stations and what are describedas separate processing stations may be combined as appropriate.Similarly, when a first element is described as being moved, analternate element may be moved and the first element may remain in placeor both elements may be moved. For example, the laser or the nitinolworkpiece or both may be moved in order to provide the local areatreatment. It should also be noted that the systems and methodsdescribed herein are also anticipated to be effective with magneticshape memory alloys such as NiMnGa.

Experiments have been conducted that have successfully modified thelocal transformation temperature of nitinol by laser treatment. Asdescribed above, the effect is believed to be primarily based onvaporization of select elements occurring due to differences in vaporpressures of each element. Also, segregation that occurs during thesubsequent re-solidification of the molten material can further alterthe local chemistry. These effects are believed to result in changes tothe local chemical composition in the re-solidified portion, and in turnalters the local transformation temperature and the shape memory effect,allowing for a single workpiece or part to possess multiple shape memoryeffects. The changes in the local chemistry can be very slight dependingon the processing parameters used.

In one experiment a neodymium-doped yttrium aluminum garnet (Nd:YAG)laser was used. Several key parameters are used to control the pulsedNd:YAG laser process. These parameters include but are not limited to:pulse width; peak power; frequency; laser movement speed (sometimesreferred to as welding speed); and defocused distance. The pulse energyand average power are also used in order to conceptualize the amount ofenergy transferred to a material. The operator presets the peak power,pulse width and frequency on a laser machine. The peak power is theinstantaneous power of the laser pulse and can influence the temperaturerise of the material. Melting is initialized when there is sufficientheat to raise peak temperatures above the liquidius temperature of theworkpiece. This process involves overcoming heat loss due to conductionand convection. The pulse width is the time each pulse irradiates theworkpiece. The larger the pulse width the longer the time the peak poweris applied. Finally, the pulse frequency is the number of times thelaser is pulsed per second, which can be used to control the amount ofpulse overlap and heat input to the workpiece. In this experiment, apulsed laser is used but this is not necessarily a requirement herein.

Laser movement speed and defocus distance are parameters that can alsohave an impact on the overall processing of a workpiece. The lasermovement speed influences the amount of overlap on each spot size for agiven pulse frequency. However the pulse frequency and laser movementspeed are typically correlated to attain the desired spot overlap. Inthe field of welding, spot overlap is typically varied from about 50%,for strength of weld applications, and 80% for applications where theweld is intended to form a hermetical seal.

FIG. 3A shows processing of a material in a manner referred to asconduction welding and FIG. 3B illustrates keyhole welding mode, whichoccur during laser processing. During conduction mode, the laserintensity from the laser beam 210 may be only sufficient to melt theworkpiece. A weld pool 220 initiates at the surface and grows due toconduction in all directions, resulting in a semi-elliptical shaped weldand heat affected zone 230. Since the laser energy is only absorbed bythe top surface of the material, material reflectivity can substantiallyreduce the amount of heat transfer.

Keyhole mode occurs when peak temperatures at the surface are sufficientto vaporize the workpiece material. A keyhole depression 240 in themolten weld pool 250 may be created from the pressure of vaporization.This results in a narrow weld with deep penetration and heat affectedzone 260, as shown in FIG. 3B. Compared to conduction welding, keyholewelding is more efficient with transferring heat to the workpiece. Thekeyhole traps the laser energy and the internal reflection within thekeyhole can act as a blackbody.

Commercially available SE508 Nitinol strip 0.37 mm thick was used inthis experiment. The chemistry for this particular alloy was 55.8 wt. %Ni and 44.2 wt. % Ti with maximum oxygen and carbon contents of 0.05 wt.% and 0.02 wt. %, respectively. The as-received cold-rolled material washeat treated for 1 hour at 800° C. to attain pseudoelastic properties. Adilute solution of hydrofluoric and nitric acid was used to remove theblack surface oxide before laser processing.

Laser processing was performed using a 400 μm spot diameter and three mspulse time. In this experiment, minimum criteria included fullpenetration and hermetic seal conditions (80% overlap). It wasdetermined that 0.6 kW peak pulse power was sufficient for producingfull penetration. Convention shows that 80% overlap of melted spots willproduce hermetic seal conditions. Table 1, below, shows the selectedparameters, variable process parameters including pulse frequency andpeak power. The parameters were selected using Equation 1, whichcorrelates the various parameters including frequency (f), spot diameter(ds), laser movement speed (V) and percent overlap (% OL).

f=100V/(d _(s))(100−% OL)  [1]

From the above equation it may be shown that the pulse frequency andlaser movement speed are directly related (i.e. higher pulse frequencyleads to higher welding speed). Hence the terminology laser movementspeed (V) will sometimes be referred to as pulse frequency (f).

TABLE 1 Selected welding parameters Welding condition (peak power,frequency) Welding speed 0.6 kW, 10 pps 48.0 mm/min 0.7 kW, 10 pps 48.0mm/min 0.8 kW, 10 pps 48.0 mm/min 0.9 kW, 10 pps 48.0 mm/min 0.6 kW, 1pps 4.80 mm/min 0.6 kW, 5 pps 24.0 mm/min 0.6 kW, 15 pps 72.0 mm/min

Tensile specimens were prepared using wire electric discharge machining(EDM) cutting in order to minimize effects of burrs during mechanicaldeformation. A transverse weld configuration was selected to investigatethe effects of both weld and base metal. FIG. 4A shows a schematic of atensile specimen 270 with dimensions; the sub-sized samples wereselected to have sufficient weld area along the gauge length. FIG. 4Billustrates the specimen with weld location 280. Tests were performedusing an Instron model 5548 micro tensile machine with a load cellresolution of ±3 N. All tests were performed at approximately roomtemperature (25° C.). Cyclic loading was conducted using a cross headspeed of 0.04 mm/min to apply a first loading cycle up to a strain of0.06 mm/mm followed by an unloading cycle down to a stress of 7 MPa. Thesame cycle was repeated 50 times (50 cycles) for both parent and laserwelded specimens. After completion of 50 cycles the specimens werestrained at a cross head speed of 0.4 mm/min until fracture.

A schematic of the stress strain curve of a loading-unloading cycle fora typical NiTi exhibiting pseudoelastic behaviour is shown in FIG. 5.The pseudoelastic parameters E1, E2 and permanent residual strain aredefined in this figure. E1 is the energy dissipated per unit volume inone complete cycle and E2 is the stored energy per unit volume onloading and available for release during unloading. The efficiency forenergy storage (η), may be expressed by Equation 2.

η=E ₂/(E ₂ +E ₁)  [2]

The dimensions of the welds were measured using metallographicprocedures. Mounted samples were ground using SiC paper withsuccessively decreasing grit size. Samples were polished using 1 μmdiamond and etched with 14 ml HNO₃, 3 ml HF and 82 ml H₂O. FIGS. 6A and6B shows the effect of pulse frequency and peak power on the minimumweld width. Minimum weld width is depicted in the schematic shown inFIG. 6A. Nominal change to the weld width was observed with increasingpulse frequency while maintaining weld power, as shown in FIG. 6B.However with increasing weld power the minimum weld width increased from260 μm to 460 μm with power increasing from 0.6 kW to 0.9 kW.

Comparisons between engineering stress-strain curves for unwelded andwelded specimens of varying pulse frequency and power input are shown inFIG. 7 and FIG. 8 respectively. Typical pseudo-elastic behaviour ofshape memory alloys was observed for the base metal specimen, indicatedby a flat region (plateau) after linear elastic straining near 0.03mm/mm strain and 290 MPa stress. Beyond 0.12 mm/mm strain, plasticdeformation of martensite occurred and the load increased due to strainhardening, followed by failure near 0.90 mm/mm strain.

FIG. 7 shows that the ductility and strength decreased significantly forthe 0.6 kW laser welded specimen with higher pulse frequency (5 pps, 10pps and 15 pps). This was due to premature failure in the weld zonebefore sufficient stress could be applied to transform the adjacent basemetal to martensite. However, a slight increase in ductility andstrength was observed at the lowest pulse frequency of 1 pps in the 0.6kW laser weld (FIG. 7). The 1 pps weldment was also able to reachstrains capable of inducing plastic deformation of martensite along thegauge length. The engineering stress-strain curves for varying weldingconditions-peak power (0.6, 0.7, 0.8 and 0.9 kW) with constant pulsefrequency (10 pps), are shown in FIG. 8. Except for the 0.6 kW weld,each of the other conditions (0.7, 0.8 and 0.9 kW) surpassed thepseudo-elastic region. However the failure strength and ductility of allwelded specimens were less than 70% and 50% of those of the base metal,respectively. The effects of welding parameters showed an increase intensile strength with increasing weld power. This reduction of fracturestrain of laser welded NiTi alloy has been attributed to several factorsincluding segregation of solute during solidification and thecoarse-grain and dendritic structure in the weld metal. However currentresults show that welding parameters can influence the mechanicalproperties; specifically, the higher energy input and lowered pulsefrequency resulted in improved mechanical performance.

FIG. 9 details the stress-strain diagrams from elastic to the onset ofpseudo-elastic deformation for un-welded and laser welded specimens fordifferent welding powers. Typical pseudo-elastic behavior of NiTi due tostress induced martensite (SIM) transformation was observed duringstraining (austenite→martensite) for most tensile specimens. However,results showed evidence of an initial yielding in the welded specimens,which became more pronounced with increasing peak welding power. Theseresults suggest an inelastic deformation occurred in the weld zoneduring straining before the usual pseudoelastic behaviour of the basemetal. During loading, the transverse weld tensile specimen inducedstress in both the base and weld metal. Hence, initial yielding mayresult from weld region, while the subsequent pseudoelastic propertiesarise from the remaining base material.

The initial yielding occurred in the weld metal, between 0.015 mm/mm and0.022 mm/mm strain; additional straining then induced transformation inthe remaining gauge length. The SIM transformation is interpreted asreflecting the base metal (BM) stress-strain curve. In FIG. 9 yieldingin the welded specimens occurred at a lower stress, which suggeststransformation occurred in the weld. The amplified definition of theyielding with increasing peak power may be attributed to the increasingweld width, as observed in FIG. 6. Increasing weld power resulted in alarger minimum weld width. Accordingly, a larger weld area within thegauge length underwent the initial SIM transformation.

It is known that deformation of the plastic deformation of the twinnedmartensite phase is irreversible when sufficient additional stress isapplied at a given temperature. In order to further detail thisdetwinning, a 2-cycle loading test was conducted at room temperature.FIG. 10 shows the first and second loading curves for the 0.9 kW, 10 ppsweld condition, which was strained up to 0.06 mm/mm. During initialloading, detwinning of the weld metal occurred which was indicated bythe yielding, followed by the SIM transformation of the base material.The second loading cycles showed the absence of the yielding, indicatingthe occurrence of irreversible detwinning within the weld metal.

Nitinol transformation temperatures have been closely linked to the SIMtransformation and can be strongly influenced by processing routes andtechniques. Remelting due to laser processing alters base metalmicrostructure, which, for nitinol, can result in the formation ofdendrites or coarse grains and segregations at grain boundaries.Furthermore, abnormal room temperature phase shifts in nitinol due tolaser treatment may also occur. These modifications to the weld metalmay be attributed to its altered transformation temperatures. It isanticipated that more detailed microstructural analysis of weld metalwill be required in order to determine all of the factors responsiblefor the altered transformation temperatures.

The variation of efficiency for energy storage (η) and permanentresidual strain with number of cycles (N) are plotted in FIGS. 11A and11B. Cyclic loading was not conducted on the 0.6 kW power laser weldedsamples with 5 pps, 10 pps and 15 pps since premature failure occurredbefore 0.06 mm/mm strain. FIG. 11A shows a rapid increase in permanentresidual strain between 1 and 5 cycles for both base and weld metal.Beyond 5 cycles each material reached a steady state. The ability of amaterial to regain its original shape after unloading can be measured bypermanent residual strain. All welded specimens showed higher permanentresidual strain compared to the BM when straining up to 0.06 mm/mm.After 10 cycles the magnitudes of residual strain for base and weldmetal were 0.020% and 0.026%, respectively. FIG. 11B shows efficiencyfor energy storage (η) as a function of cycles (N). Both base and weldedmaterials showed an increase in η up to 5 cycles. Weld material showed aslightly improved efficiency during the first 5 cycles. Beyond 20 cyclesthe efficiency stabilized near 0.9%. Hence compared to the base metal,welded specimens showed higher overall permanent residual strain andexhibited slightly higher efficiency for energy storage during theinitial 5 cycles.

As detailed earlier, the initial yielding occurred in the weld metal,resulting in a cold worked weld region. Therefore the increase inpermanent residual strain of welded specimens could be due to thepermanent SIM transformation after initial loading. In addition slightincrease in permanent residual strain in the specimens made at higherpower input can be attributed to the increased weld width. It has beenshown that improved n values can be attained by cold working TiNi SMA.Hence the improved efficiency for the welded specimens during theinitial cycles may be attributed to the plastic deformation of the weldmetal after the initial cycle where inelastic deformation was induced.

Failure occurred within the weld zone of the tensile specimens for eachwelded specimen. Base metal fracture surfaces revealed a dimpled surfacesuggesting ductile fracture. The fracture surface of the 0.6 kW and 1pps weld condition exhibited lowest tensile strength. A smooth fracturesurface showing the directional dendritic solidification structure ofthe weld was observed. This is indicative of transgranular failure wherefracture propagates at the dendrite interface. In contrast, the 0.9 kW,10 pps welding condition revealed a relatively coarser surface. Whenobserved closely a finer dimpled structure was exposed, suggestingductile intergranular failure through the fusion zone dendrites. Theseresults reveal that changing welding parameters can result in differentfailure modes; however, it is suggested that further research detailingweld microstructure is required to determine the mechanism responsiblefor this failure mode transition.

FIG. 12 shows the hardness trace of the weld cross-section. Along thex-axis, all samples exhibited a decrease in hardness within the fusionzone. Hardness values increased gradually away from the weld centerlinebefore finally converging to that of base metal. Base metal hardnessvalues ranged near 370-400 Hv. Minimum weld hardness was observed in the0.6 kW, 10 pps condition, which approached 250 Hv. In contrast the 0.6kW, 1 pps and 0.9 kW, 10 pps weld conditions exhibited minimum hardnessvalues near 280 Hv. Lower hardness in the weld centre of the previouslyannealed materials may be attributed to resolidification induced bywelding, which can result in larger near strain-free recrystallizedgrains. However the primary reason softening was experienced may be dueto the local phase change into the softer martensite at roomtemperature.

Hardness values along the y-axis of the weld centerline, shown in FIG.12, were similar among samples. Hardness values for the 0.6 kW, 10 ppsweld bottom showed slightly lower hardness values when compared to theweld surface. Hardness for the 0.6 kW, 1 pps weld was relativelyscattered across the centerline, similar to the pattern in thelongitudinal direction. However, 0.9 kW, 10 pps showed relativelyconsistent hardness values in the transverse direction.

FIG. 13 shows the differential scanning calorimetry curves for the baseand weld materials. Both austenite finish (Af) and martensite start (Ms)temperature were below room temperature, −8.61° C. and −33.27° C.respectively. This indicates room temperature phases were predominantlyaustenite, hence the presence of pseudoelastic behaviour during tensiletesting. The weld material exhibited similar thermal events as the basematerials; however a pair of higher temperature peaks was also present.

An additional peak is typically observed in cold worked or aged Nitinolduring R-phase transformation. However in the instance of R-phasetransformation, an intermediate martensitic transformation, it wouldproduce one peak between austenite and martensite during cooling and thepresent weld material shows two distinct transformation peaks outside ofthat range. In addition, the fully annealed base material did not showany presence of R-phase transformation, due to preserving of the solidsolution by quenching to roughly room temperature. Hence theseadditional peaks suggest the presence of multiple phase transformation,including a low temperature (<room temperature) and high temperature(>room temperature) transformation. Quantified peak onsets are providedin Table 2.

TABLE 2 Peak onset for DSC scans Low Temperature Transformation HighTemperature transformation A_(s) A_(f) M_(s) M_(f) A_(s) A_(f) M_(s)M_(f) Base Metal −16.1 −8.61 −33.27 −44.23 Not Present 0.6 kW, 10 −20.17−14.79 −37.72 −48.83 70.96 89.34 62.63 22.07 pps 0.6 kW, 1 −21 −16 −39.3−47.17 52.08 96.75 66.07 24.02 pps 0.9 kW, 10 −24.45 −20.98 −43.29−52.99 67.56 94.02 64.47 29.3 pps

Optical micrographs were completed of the weld cross-section. Weldsshowed the typical banded structure created during each thermal cycleduring the pulsed Nd:YAG process. The use of polarized light aided indefining the segregated phases that were shown to be concentrated nearthe weld surface. Possible variances in cooling rates experienced alongthe vertical plane of the workpiece during the pulsed Nd:YAG weldingprocess can result in the top surface to be the last region to solidify.In turn this can promote the formation of intermetallic phases near thetop surface of the weld. However detailed thermal analysis (usingthermocouples) is required to determine the presence and magnitude ofcooling rate gradients.

Base metal and fusion boundary microstructure are shown in FIGS. 14A and14B, respectively. As expected the annealing process resulted in largergrains that from DSC results are shown to be austenite NiTi at roomtemperature.

FIG. 14B shows representative fusion boundary microstructure for the 0.9kW, 10 pps weld, which is located at the interface of the remolten andbase material. Columnar dendritic growth was observed near the fusionboundary. Narrow heat affected zones (HAZ) are inherent to the pulsedNd:YAG process due to its low heat input, consequently the HAZ isindefinable in FIG. 14B.

The fusion zone microstructure for each weld condition was alsoobserved. Each condition had varying amounts of continuous submicronsegregation. The 0.6 kW, 10 pps weld showed a high density of continuousintergranular segregation. In contrast, the 0.6 kW, 1 pps weld showed arelatively lower density of similar segregation. However, intermittentsecond phase distribution was observed for the 0.9 kW, 10 pps weld.Segregated phases in the fusion zone can act as preferential sites wherefailure initiates or propagates. The varying amounts of segregation canbe correlated to the weld mechanical performance shown in FIG. 7 andFIG. 8. The densely segregated 0.6 kW, 10 pps welds exhibited thepoorest mechanical performance while the intermittently segregated 0.9kW, 10 pps weld showed to have relatively better performance.

Room temperature XRD data showing indexed peaks for the base metal, weldsurface and weld bottom for all conditions are shown in FIG. 15. Basemetal peaks distinctly identified the sole presence of austenite, asexpected from BM DSC result in FIG. 13. All weld condition showedevidence of both austenite and martensite phases on the weld surfacewhere high concentrations of segregated phases were present. The weldbottom of each weld exhibited differing types and relative amounts ofphases. The 0.6 kW, 10 pps weld exhibited only austenite phase while the0.6 kW, 1 pps weld showed austenite and some evidence of martensite.However the 0.9 kW, 10 pps weld showed both austenite and martensitephases similar to its top surface, which further corroborate thehardness trends in 0.9 kW, 10 pps. Hence, these results suggest weldingparameters can result in different phases at the top and bottom of eachweld.

High temperature DSC peaks observed within the welded sample can beassociated with the martensite phases observed in the XRD results.Quantified peak onsets, shown in Table 2, suggest the Ms temperature forthis phase to range 60-67° C. Hence, the chemistry of the martensiticphase observed in the weld metal may result from equiatomic or Ti-richchemistry. This in turn implies the observed segregated phases in thefusion zone possibly being Ti-rich intermetallics, of which Ti₂Ni ismainly observed. However XRD analysis was unable to detect the presenceof these intermetallics, likely due to the lack of grain populationrequired to produce a detectable XRD signal. Hence detailedmicrostructural observations (including TEM) are required to identifyand characterize the submicron segregated phases within the weld metal.

The experiments investigated the mechanical properties of pulsed Nd:YAGlaser processed nitinol. The weld strength, pseudoelastic and cyclicloading properties for varying parameters were compared with the basematerial and fracture surfaces were analyzed. In addition, selectwelding conditions were analyzed using hardness testing, DSC scans,metallographic examination and XRD analysis. Key observations included:

-   -   1) Processing parameters (peak power and pulse frequency) were        shown to strongly influence the mechanical properties (tensile        strength and ductility) of the micro laser treated NiTi alloy.        Higher peak power and lower pulse frequency resulted in improved        mechanical performance.    -   2) Evidence of initial yielding was observed in welded specimens        during transverse tensile loading. Yielding resulted from        detwinning occurring in the welded region during tensile        deformation (weld and base metal).    -   3) Laser processed samples showed higher permanent residual        strain and exhibited a slightly higher efficiency for energy        storage during the initial 5 cycles compared to base material.    -   4) Multiple phase transformations were observed in fusion zone        DSC scans. These transformations occurred at low (below room        temperature) and high temperatures (above room temperature).    -   5) Microstructure observations showed large austenite grains in        the annealed base material and columnar dendritic growth was at        the fusion boundary. The 0.6 kW, 10 pps treatment had high        amounts of segregation and failure occurred premature to the        pseudoelastic region. In contrast the 0.6 kW, 1 pps welds showed        intermittent segregation and exhibited better mechanical        performance.    -   6) XRD results showed that the weld metal contained both        austenite and martensite phases at the surface for all        conditions. However, the weld bottom showed the austenite phase        with varying amounts of martensite, which depended on the weld        condition.

Although a pulsed Nd:YAG laser was used in the noted experiment, it willbe understood that other sources of localized energy/heat can alsoachieve similar results. In the case for lasers, a continuous wave laserinstead of a pulsed laser may also be applied. This may include, but isnot limited to, diode, fiber and carbon dioxide lasers.

FIG. 16 illustrates two discrete memories embedded in a single nitinolribbon as a result of the above experimentation. FIG. 16A shows adeformed “C” shape that can be heated and transformed to the firstmemory shape shown in FIG. 16B; additional heating results in thecomplete transformation and the final memorized shape shown in FIG. 16C.

FIG. 17 illustrates the potential application of multiple transformationtemperatures to 2-D (FIG. 17A) and 3-D (FIG. 17B) configurations. Inthese examples, zones of differing transformation temperatures are shownwith different shades of grey. It will be understood that various shapesmay be obtained by the use of these different transformationtemperatures. In particular, the temperature ranges are dictated by whatis desired and the material being used, for example approximately −150to 150 degrees Celsius for NiTi, and higher or lower for other alloys.

FIG. 18 illustrates a potential application of the differingtransformation temperatures for an actuator device 300. In this example,a central arm 310 of a three-armed actuator device 300 can be treated tohave a differing transformation temperature than the outer arms 320 and330. These differing transformation temperatures will allow for the useof the actuator device 300 in a two-stage actuation. In a furtherembodiment of the method above, the actuator device 300 may be furtherheat treated (annealing or the like) in order to alter the localstructure and chemistry to create a gradient of Ni concentration alongthe central arm 310, thus providing a gradient of transformationtemperatures along the central arm 310. As a rough example, if theactuator device 300 is initially treated to have 51 atomic percent Niconcentration and a first transformation temperature, the central armmay be treated to have a 49 atomic percent Ni concentration and a secondtransformation temperature. The subsequent heat treatment can result indiffusion of Ni atoms into the central arm to provide a concentrationgradient along the central arm 310, and thus a gradient oftransformation temperature to provide a smooth actuation.

It will be understood that the additional transformation temperature(s)imparted to the material will depend on starting parameters as well asprocess parameters. As such, the starting and process parameters (e.g.the range of the local heating/melting needed) can be varied to tailorthe transformation temperature. The transformation temperature availableis not limited to those temperatures used in medical devices or the likebut is only limited by the properties of the shape memory material inuse.

It is anticipated that additional techniques may also be used or assistwith modifying local structure and chemistry in order to fine tune thetransformation temperatures and zones/areas. This includes using variousheating processes to induce melting such as: Laser re-melting; Micro-Arcre-melting; Resistance melting; and the like and can be implementedeither individually or in some combination. In particular, it ispossible to adjust the energy source that is applied to the material toadjust the local structure and chemistry.

An alternate technique also includes using additional material or asecond material, or filler material, with a varied composition, which isthen included in the material as a part of the laser process or using ajoining technique. Examples of filler material may include pure nickel,pure titanium, palladium and platinum. Joining methods can include solidstate diffusion bonding/brazing, laser welding; arc welding; resistancewelding and the like. In some cases, it is expected that shape memorymaterials having different transformation temperatures can be bondedtogether through the addition of energy (for example, using theprocesses herein) to produce a monolithic shape memory element having,for example, a third transformation temperature at the bonding site.

Another aspect of the changing of local chemistry to provide theformation of multiple transformation temperatures (memories) to a shapemetal material is that the material will thereby have a stress-straincurve that reflects multiple pseudo-elastic regions. FIG. 19 illustratesthe type of stress-strain curve expected for a strip of shape memorymaterial having multiple transformation temperatures along its length.As shown, for the loading curve, the material will be expected toexhibit multiple sequences of elastic deformation followed by a plateauof pseudo-elastic deformation. The unloading curve is expected to besimilarly affected.

One of skill in the art will understand that the processes and systemsdescribed herein can be applied to other SMAs and SMPs with appropriatemodifications. For example, when dealing with SMPs, the range oftemperatures and times (i.e. pulse frequency and the like) needed willbe different and alternate energy sources or techniques may be used toadjust the local structure and chemistry of a local area of the SMP toprovide a similar effect.

The methods and systems herein can be applied to various industrialapplications and unique solutions can be implemented to addressparticular applications. An example of a current application includesSMA actuators. Current SMA actuators typically require a bias whichretracts the SMA material back to an original position. The bias iscommonly facilitated using a conventional spring. However, if an SMAhaving multiple transformation temperatures is used, the use of a biasmay be eliminated.

A shape memory material that has multiple transformation temperaturescan be used in various applications where an object needs to react todifferent temperatures and/or there is a need to gradually adjust theshape of the object. In particular, multiple transformation temperaturesallow for applications where rather than just an open or closed shape ofa particular metal, there can be a gradual opening or closing based onthe temperature applied to the object. Examples might include: valves,such as flapper valves, diaphragms for medical or industrialapplications, sensors for temperature or monolithic actuators withmultiple transformation points, micro-grippers, stents and MicroElectro-Mechanical Systems (MEMS). As one particular example, multipletransformation temperatures would allow for the construction of tubesthat can be expanded and then connected by heating. One end of the tubecould be formed larger and then heated to contract to a smaller memoryshape in order to bond to another tube member.

Further, the present methods and systems allow the processing ofpre-fabricated, commercially available parts to add additionaltransformation temperatures, which reduces production costs whencompared to techniques such as tape casting or LENS that must create anentirely new part. Further, titanium oxidation is also avoided sinceelemental titanium is not required in the present process. Stillfurther, the final product is generally porous free with mechanicalperformance that is essentially the same as the single transformationtemperature monolithic shape memory material. Still further, the productmay also have lower weight.

FIG. 20 illustrates an example application of a shape memory materialhaving multiple transformation temperatures. In particular, FIG. 20shows a diaphragm of a type that might be used in various applications.In this example, a central area 400 has a different transformationtemperature, higher than room temperature, than a supporting frame 410.This allows the central area 400 to be deformed separately at roomtemperature from the supporting frame 410 creating the diaphragm shape.Multiple transformation temperatures are important in this situation inorder to allow the central area 400 to be in an elastic orpseudo-elastic state while the supporting frame 410 remains in anon-elastic state.

FIGS. 21, 22A and 22B illustrate other examples of the industrialapplication of a shape memory material having multiple transformationtemperatures. In this case, the application is for a valve. FIG. 21illustrates a first example valve in which the valve may be mounted atan edge of an inlet or outlet and the multiple transformationtemperatures can be used to open a flow pathway at two (or more)different levels. More specifically the valve contains a flapper arm,which has been embedded with multiple memories. The flapper arm issecured at one end and the other end is positioned to cover a fluid flowpassageway. In a first position the flapper arm will restrict all ormost of the fluid from flowing through the fluid flow passageway. Thisflapper arm will respond to these multiple memories by changing shape toallow more or less fluid flow through the fluid flow passageway,depending on the desired response at a given temperature. This flapperarm arrangement may be used in heat exchangers to modulate fluid controlbased on the temperature of the fluid.

FIGS. 22A and 22B illustrate an example valve in which the valve isshaped as a dome and sections of the dome are formed such that thesections may open to allow flow through the valve in various directions.The sections of the dome will typically be formed at differenttransformation temperatures but this may depend on the required flowpatterns/pathways. It will be understood that the dome may be mounted inthe flow pathway in various ways, including friction (i.e. sandwichedbetween plates or within a tube or the like), bonding, fasteners or thelike. Valves are used in many applications throughout industry. Onespecific example is the use of a valve to redirect the flow of enginecoolant in the automotive industry. For example, when an engine starts,engine coolant should not travel through a heat exchanger until it ishot enough to need to be cooled. As such, a temperature operated valvecould be very convenient in redirecting engine coolant flow.

In ongoing studies of the process and system described herein, otheraspects of the process and system have also become apparent. Forexample, the local change in composition of the material being treatedis also expected to provide an improvement in corrosion resistance. Arobust oxide layer is critical in achieving corrosion resistance and arobust oxide layer can be achieved when an oxide stabilizing element ispresent. In the case of NiTi, the titanium-richer alloy has a higheraffinity towards oxidation however is less likely to form one in thepresence of excess nickel. For example, TiO₂ (or even NiTi₂O₄) oxidesform when there is sufficient titanium present. However, in a typicalNiTi system, the near-equiatomic composition is often slightly moreNi-rich to take advantage of the pseudoelastic properties of the roomtemperature austenite phase. Furthermore, this pseudo-elasticity is theprimary functional property exploited in medical device applications. Asa result there is a current drive in research to develop anunderstanding of the corrosion properties of Ni-rich nitinol.

Since the shape memory material process described herein locallymodifies the chemical composition, this can also result in a change inlocal corrosion resistant properties. More specifically, a reduction inconcentration of Ni (and consequently an increase in Ti content) at thesurface of a nitinol workpiece results in the formation of a more robustoxide layer and improved corrosion resistance. Some of the benefits ofapplying the shape memory material process for corrosion resistanceinclude, but are not limited to:

1) Bulk material properties can remain the same with only the surfacebeing modified to achieve improved corrosion performance. For example,the pseudoelastic properties of Ni rich NiTi can be retained while thesurface exhibits properties similar to Ti rich alloys.

2) Select locations can be treated in a workpiece. For example, in acase where a galvanic coupling may be made with another component,processing can be implemented to create a resistant interface.

3) The depth of penetration can be precisely controlled by adjustinglaser pulse frequency and duration of treatment, potentially making theprotective layer much more robust when compared to coating technologies.The depth of penetration may depend on the laser power density, which isimproving in the industry. With current technology of depth of 50 mm maybe achievable, if not more and a minimum of tens of microns is likelyachievable, although smaller minimums in the range of nanometers mayalso be possible.

In particular, through the processing method described above, theelement with the higher vapor pressure is vaporized and removed,increasing the concentration of the other element on the surface andadjusting the local chemistry at the material surface. Also, byadjusting the thickness of the oxide levels at the surface by adjustingthe depth of treatment, the optical properties may also be manipulatedthrough electro polishing, a process known in the art. Althoughdescribed in terms of NiTi material, it should be understood that theprocess may be applied to other materials, where there is a plurality ofelements in the material and, thus, a difference in vapor pressurebetween the elements.

As described above, during processing, peak temperatures surpass themelting point and upon cooling solidification occurs. Surface morphologycan be controlled based on the experienced solidification rate. Surfacetextures (or even roughness) can include smooth (achieved with slowcooling rate), rippled (intermediate cooling rate), or porous (entrappeddue to rapid cooling rate). Furthermore, the interaction of variousthermal cycles can further enhance the surface morphology to attain adesired texture. Some of the advantages associated with this include:

1) Localized processing can enable only a desired area to be treated.Furthermore, multiple surface textures can be embedded in a singlecomponent, or even a gradient of surface textures.

2) When combined with other process results (i.e. corrosion resistance,shape memory) with altered surface textures, the local area can befurther tailored, for example. in addition to porous, the surface willbe softer and more corrosion resistant.

3) The depth of processing can be controlled relatively precisely(speculated to range from tens of microns to centimeters) with minimaleffects to the bulk material.

One of the benefits of these modifications may be the enhanced surfacesfor bone or cell growth for medical device applications.

Another aspect of the processes and systems herein relates to theremoval of contaminants from materials. During the manufacturing processof materials, and in particular, alloys, contaminants may be present inthe raw material or enter the material during the manufacturing process.For example, NiTi alloys may contain carbon or other contaminants. Insome cases, the contaminants can also result in the formation ofintermetallics (for example, TiC)., which consume elements from the bulkmaterial and can change the overall chemical ratios. As such,contaminants may make it more difficult to attain a desiredtransformation temperature. Furthermore, degradation of mechanicalperformance can occur (i.e. fatigue experienced due to stress risers).By using the described processing systems and methods, contaminants maybe successfully removed and a purer alloy can be attained in theprocessed region. This result has been observed when embedding anadditional memory in NiTi using the shape memory material process. FIG.23 shows a cross-section of a processed region in the centre and bulkmaterial at the edges. In this example, the dark spots in the bulkmaterial are believed to be TiC or other contaminants. Followingprocessing, the contaminants are reduced or removed, as shown in FIG.23. It is believed that the contaminants are vaporized during theprocess and a purer bulk material results. As shown in FIG. 23, as thecontaminants are removed, there may be a slight volume change in thebulk material. Again, NiTi alloys are used as the example, however itshould be understood that similar results may be achieved for otherappropriate materials using the methods and systems herein.

In both the corrosion resistance treatment and in the removal ofcontaminants, the depth to which a material may be treated willgenerally depend on the power associated with the laser, the chemicalproperties of the material and its components, and the like. Currenttesting has shown that it is possible to process up to a depth ofapproximately 50 mm for NiTi, but this depth may be different for othermaterials and may increase as more powerful lasers are used or becomeavailable.

In still another aspect of the methods and systems herein, it has beendetermined that the process can also be used to cause a strengthening ofcertain types of materials. In particular, in the case of NiTi alloys,the formation of small second phase (Ti₂Ni) particles was observed afterthe removal of nickel by using the process herein. FIG. 24 shows atransmission electron microscope (TEM) image of a pair of particlesroughly 100-150 nm is diameter. The base material, having the martensitephase twin structure, is also visible in FIG. 24. It is believed thatthese second phase particles can further enhance the properties of NiTialloys and other shape memory materials through the creation of multiplenucleation sites. There are at least two mechanisms believed to aid inenhancing the properties which are as follows:

1) The second phase particles can act as precipitation strengtheningpoints, much like composites strengthen composite materials (or evendual phase steel); and

2) During solidification, these second phase particles can act asinoculants that promote nucleation of grains and may result in a finergrain structure (making the material stronger).

Evidence of the smaller grain structure is shown in FIGS. 25A to 25D. Inthese figures, it should be noted that the etchant used attacks thegrain boundary, and the darker regions indicate preferential attack ofgrain boundaries in the small-grain regions. Also, as the number ofapplied laser pulses increases, the amount of nickel removed alsoincreases. FIG. 25A shows the situation after a first pulse and thegrain size does not show a significant change; however, after the 2^(nd)pulse, FIG. 25B, and 3^(rd) pulse, FIG. 25C, the structure may includean increasingly finer grain structure. Upon closer examination of the 10pulse sample FIG. 25D, the Ti₂Ni phase was observed. Furthermore, therapid cooling experienced during solidification may inhibit graingrowth. Hence the formation of the second phase may promote a finergrain structure within the processed region. There may also be somechange in mass, due to the process used, but it is unlikely to begreater than 10 weight percent. In many materials the change in atomicmass is unlikely to be greater than 2 weight percent.

The effects of changing composition (for example, becoming more Ti-rich)on the microstructure can be predicted by examining the partial binaryNi—Ti phase diagram near the equiatomic region, as shown in FIG. 26.Assuming the cooling path labeled C₀ represents the original bulkcomposition of the alloy (nearly 50.7 at. %), as the composition becomesmore titanium rich, the solidification range decreases until a congruentsolidification is attained at the equiatomic composition (C₁). Furtherdecrease in Ni from C₁ to C₂ may result in a drastic increase insolidification range (from 0 to about 300° C.) until the eutectictransformation occurs at 984° C. Compositions with Ni contents below C₂stabilize into a dual-phase or multiple-phase structure, which includesNiTi and Ti₂Ni below the eutectic temperature. Rapid cooling experiencedfrom the shape memory material process may increase the Ti₂Ni nucleationsites and may result in finer grains or particles as observed in FIG.26. Again, this result may occur in other alloy systems and it should beunderstood not to be limited to NiTi. In particular, this finer grainstructure is expected to be applicable to any material that willnucleate at least one additional phase during solidification.

In applying the embodiments of the systems and methods herein, it willbe understood that various combinations may be used. In some cases, itmay be appropriate to treat a predetermined portion of a material, suchas for adding a memory to an SMM or for treating a surface of thematerial, while in others it may be appropriate to treat a predeterminedportion that includes all of the material, for example, when removingcontaminants from a material. Further, the embodiments herein may beused to treat a single material or to bond one or more materials(potentially including filler materials) while controlling localchemistry at the bonding site.

As noted above, a multi-memory shape memory alloy, and in particular,one made using the processes described herein, may have application in awide variety of areas, including providing improved functionality inexisting devices and, in some cases, enabling the development of devicesthat may not have been possible using conventional technology. In orderto provide some example, current devices that may benefit frommulti-memory shape memory material technology include, but are notlimited to:

1) Diaphragm: A multi-step diaphragm may now be constructed takingadvantage of the two or more discrete memories that may be embedded inthe shape memory material. Diaphragms may be used in, for example,aerospace applications.

2) Actuator: A monolithic actuator may take advantage of the shapememory and pseudo-elastic properties of nitinol, both of which can beimparted in a monolithic Nitinol device using the multi-memory shapememory material technology. There is a need for these actuators in MEMSapplications.

3) Automotive tensioner: An automotive tensioner may be able todynamically change the tension of a timing-belt to prevent slippage andpower loss as the engine heats up. This application would ensures thecrankshaft and camshaft are timed correctly through a wide temperaturerange.

4) Valve: A multi-step valve, which can precisely control fluid flowaccording to thermal condition is explained above.

5) Multistep stent: A multi-step stent for medical use can also bedesigned. This would provide improved functionality and in some cases,the expansion of the stent may even be remotely controlled, possiblythrough the use of ultrasonic heating or the like. For multistep stents,it is envisioned that the well-known hysteresis relation between coolingand heating in shape memory alloys can be exploited to induce shapememory effect. For example, shape memory alloys often have an offsetbetween heating and cooling transformation temperature, which in thecase for NiTi can be up to 50 degrees. In the case of implantablestents, the operating environment is near body temperature (i.e. 37degrees Celsius). Hence a multi-step stent can be created whichgradually opens by remotely heating the device using an external energysource to slightly above body temperature (i.e. 39 degrees Celsius).This heat would be applied temporarily so as not to hurt the patient.Upon removal of heat the stent will not close unless the temperaturedrops substantially (10-50 degrees for NiTi), in which case thetemperature change would be fatal for a patient. Similarly, when using amagnetic SMA a stent may be implemented with multiple memory impartedand a magnetic field applied to achieve a similar result.

The aforementioned devices are only a sampling of the type ofapplications envisioned that may make use of the methods and systemsdescribed herein.

It should be understood that various modifications can be made to theexample embodiments described and illustrated herein as will beappreciated by one of skill in the art.

1. A method for treating a material comprising: applying energy to apredetermined portion of the material in a controlled manner such thatthe local chemistry of the predetermined portion is altered to provide apredetermined result.
 2. The method of claim 1 wherein the applyingenergy comprises processing the predetermined portion with a laser. 3.The method of claim 2 wherein the processing the predetermined portionwith a laser comprises: selecting a power, beam size, and movement speedfor the laser to produce the predetermined result; focusing the laser ona subset of the predetermined portion; and adjusting the spatialrelationship of the laser and the material such that a beam from thelaser contacts all of the predetermined portion.
 4. The method of claim2 wherein the laser is operated in a pulsed fashion to provide shorterbursts of energy to control the application of energy.
 5. The method ofclaim 1, wherein the applied energy is controlled to reduce conductionoutside the predetermined portion of the material.
 6. The method ofclaim 1, wherein the material is a shape memory material and thepredetermined result is to provide an additional memory to thepredetermined portion of the shape memory material.
 7. The method ofclaim 1, wherein the predetermined portion is the surface of thematerial and the predetermined result is to adjust the concentration ofcomponents of the material to allow the formation of an oxide layer onthe surface of the material to provide corrosion resistance.
 8. Themethod of claim 1, wherein the predetermined result is to removecontaminants from the material.
 9. The method of claim 1, wherein thematerial is a shape memory material and the predetermined result is toalter the pseudo-elastic properties of the shape memory material. 10.The method of claim 1, wherein the predetermined result is to generateat least one additional phase particle in the material to provide anucleation site for grain growth.
 11. The method of claim 1, furthercomprising cooling the predetermined portion at a predetermined rate toalter the surface texture of the predetermined portion.
 12. The methodof claim 1, further comprising adding a filler material such that thefiller material is available during the application of energy.
 13. Themethod of claim 1, wherein the material comprises two pieces of shapememory material and the predetermined portion comprises an area wherethe two pieces are to be bonded and the predetermine result comprisesproviding a transformation temperature to the predetermined portion thatis different from a transformation temperature of at least one of thepieces.
 14. A shape memory material comprising at least twotransformation temperatures wherein at least one transformationtemperature is applied following formation of the material.
 15. Theshape memory material of claim 14, wherein at least one of the at leasttwo transformation temperatures are formed by the method of claim
 1. 16.A system for treating a material comprising: an energy module forapplying energy to a predetermined portion of the material; a positionmodule for positioning the material and energy module in relation toeach other; a processing module for controlling the position module andenergy module to treat the material such that the local chemistry of thepredetermined portion of the material is altered to provide apredetermined result.
 17. A method for treating a material comprising:applying energy to a predetermined portion of the material in acontrolled manner such that the local chemistry of the predeterminedportion is altered to provide at least one result selected from among:where the material is a shape memory material, to provide an additionalmemory to the predetermined portion of the shape memory material or toalter the pseudo-elastic properties of the shape memory material; wherethe predetermined portion is the surface of the material, to adjust theconcentration of components of the material to allow the formation of anoxide layer on the surface of the material to provide corrosionresistance; to remove contaminants from the material; and to generate atleast one additional phase particle in the material to provide anucleation site for grain growth.
 18. The method of claim 17, furthercomprising cooling the predetermined portion at a predetermined rate toalter the surface texture of the predetermined portion.